1. Field of the Invention
The present invention relates to ink jet printing apparatuses, image processing apparatuses, and methods for processing an image, and more specifically, to a process for, when carrying out printing using a black ink and color inks, thinning color ink dots adjacent to a black ink dot.
2. Description of the Related Art
Ink jet printing apparatuses, which eject ink to a print medium for printing, can execute high density and high speed printing on various print media. The ink jet printing method has many advantages, for example, low running costs and reduced noise involved in a printing operation. The ink jet printing apparatus has thus been widely used and commercialized as various image output apparatuses such as printers and portable printers.
A known printing method for the ink jet printing apparatus is what is called a serial method according to which a carriage carrying print heads and ink tanks is scanned over a print medium, while inks are ejected to the print medium to print the corresponding area. This method conveys the print medium in a direction substantially orthogonal to a scan direction by a predetermined amount during the period between scans. The scan operation and the operation of conveying the print medium are repeated to print the entire print medium. Another known printing method is what is called a full line method. This method uses a print head having ejection openings arranged within the range corresponding to the width of the print medium and conveys the print medium with respect to the print head for printing.
In recent years, the ink jet printing apparatus, having various advantages such as those described above, has been provided as various products that use a plurality of colors of inks to execute color printing. In general, the color ink jet printing apparatus often uses the black ink to print characters or the like. This results in the need for a black ink that enables sharp, clear, and high optical density print characters to be printed. Some known techniques use a black ink exhibiting a reduced permeability to the print medium to inhibit a coloring material such as a dye or pigment in the black ink from permeating the print medium. This increases the amount of the coloring material remaining and fixed on the print medium to provide sharp and high density print images.
For the color inks, when inks in different colors are applied to the respective adjacent areas on the print medium, a phenomenon (breeding) may occur in which these inks mix at the boundary between the areas to degrade the quality of the resulting color image. To prevent this, known techniques use inks with an increased permeability to a print medium (see, for example, Japanese Patent Laid-open No. 55-065269 (1980)). This makes it possible to prevent much of the inks applied to the respective areas from permeating the print medium and spreading across the boundary to mix together.
However, the above combination of the black ink and color ink raises the following problem. In an image having a black area and a color area which are adjacent to each other, the black ink, having a lower permeability, may spread across the boundary between the areas to mix with the color ink instead of permeating the print medium quickly. That is, bleeding may occur at the boundary between the black and color areas.
To deal with this problem, several conventional measures have been proposed.
A first measure is to provide fixing means such as a heat fixing device. This enables the ink to be quickly fixed to the print medium to prevent possible bleeding. This also makes it possible to prevent the image from being smeared with ink not having been fixed.
A second measure is to apply the color ink, having a higher permeability, to the same area to which the black ink is to be applied. The black ink is thus applied to the sheet surface wetted with the color ink. This facilitates the permeation of the black ink through the print medium to prevent possible bleeding at the boundary between the black area and the color area. Further, the fixation is also promoted to enable the inhibition of possible smear. An image printed with the black ink is subject to smear in some areas and to boundary bleeding in other areas. The color ink is thus applied to the respective areas to promote the fixture. Moreover, the amount of color ink applied to inhibit possible smear is different from that of color ink applied to inhibit possible boundary bleeding. This promotes the fixture in each area to enable the inhibition of both smear and boundary bleeding.
A third measure is to use a combination of inks of the type in which the black ink and color ink react and coagulate with each other (see, for example, Japanese Patent Laid-open No. 9-025442 (1997)). This enables the inhibition of possible boundary bleeding.
However, the first measure provides the fixing means, requiring an increase in the size of the apparatus and in costs. Further, with the serial printing apparatus, the intermittent feeding (conveyance) of a print medium may make cause uneven fixing. Moreover, the fixation requires a somewhat long time, increasing the amount of time until sheet discharging. This unfortunately degrades throughput.
With the second measure, when the color ink, having a higher permeability, is applied to overlap with the black ink in order to facilitate the fixation of the black ink, the black ink may permeate the print medium at the same time when the color ink permeates the print medium. This reduces the black ink remaining on the surface of the print medium. Unfortunately, this in turn prevents the realization of a sharp and high density black image, which is the original purpose of the black ink. Moreover, in the area in which the black and color inks are adjacent to each other, when the color ink in the area adjacent to the black ink permeates the print medium, the adjacent black ink also permeates the print medium. In this case, the black ink may permeate the print medium to the area of the color ink, resulting in the bleeding of the black ink.
The third measure coagulates the black and color inks together. This disadvantageously limits the compositions of the black and color inks. For example, one of the inks needs to be cationic, while the other needs to be anionic; the available inks are limited.
A known technique different from those described above distinguishes black pixels from color pixels in an image to be printed and thins the color pixels adjacent to the black pixel (see Japanese Patent Laid-open No. 6-135015 (1994)). This makes it possible to inhibit possible boundary bleeding. This technique can inhibit possible boundary bleeding without raising any problem as in the above first to third embodiments.
However, the conventional technique for thinning data from adjacent color areas thins all of the color inks to be applied to areas adjacent to a black area. Thus, in spite of the successful inhibition of possible boundary bleeding, in an image with varying a gradation, a gradation skip in which the gradation varies relatively rapidly may occur. This may degrade image quality. More specifically, the above conventional thinning method thins color data even if a black area is present with color data present only in one pixel adjacent to the black area. This may degrades an image intended to express a specific gradation by providing such isolated color data. This may in turn result in clearly observed image quality degradation such as the gradation skip.